The present invention relates to diciphering elastic wave scattering by obstacles in continuous media, and more particularly pertains to a new and improved apparatus and method for deciphering the resonant echoes from fluid-filled cavities in solids so as to identify the material composition of the filler fluid.
In the fields of material science and physics, it is often important and desirable to identify the composition of inclusions contained within various materials. For example, in medical ultrasonics it is important to determine the material composition of some fluid-like substances, e.g., tumors, contained in animal tissue by ultrasonically scanning these "in vivo" with monitored sound projectors. In the area of non-destructive testing and materials evaluation, a basic problem of daily routine occurrence is to determine if a material obstacle is contained within another "host" material, and if so, what kind of obstacle it is. Explosive grains, bombs, etc., are routinely tested non-destructively (acoustically or with x-rays) to determine the presence of material imperfections, flaws, or inhomogeneities in them. In geophysics, seismic waves scattered by fluid-filled caverns in the earth's crust, carry information about the material composition of the cavern's contents. The Army has used these to determine the presence of silo-sites in foreign countries.
The search for a satisfactory method of both locating a cavity and non-destructively determining the composition of the same has recently intensified due to world-wide energy shortages. Obviously, a perfected process might be utilized to locate and identify oil, water and gas deposits contained in cavities within the earth's interior.
Acoustical analysis has long been recognized as one of the more desirable techniques of locating obstacles contained within solid materials. There have been numerous methods developed for analyzing the elastic wave scattering caused by obstacles in nonabsorbing media. A good part of the prior art methods have utilized spherical geometry, and most of the incident wave-types considered have been continuous wave signals, although some transient result methods are available for continuous wave excitations. The cases of pure plane p-wave incidence or pure s-wave incidence have been studied in some detail. The obstacle has been taken to be an elastic sphere of properties different from those of the medium into which it is embedded, a rigid inclusion, a fluid-filled cavity, an evacuated cavity, or more general situations. Other more general cavity shapes from spheriodal or ellipsoidal, to completely arbitrary have also been analyzed, and general matrix theories for elastic wave-scattering situations have recently been developed.
Some of the prior art has analyzed the stress and displacement fields, or the stress concentrations which dynamically develop in the elastic material as a result of the wave scattering by the cavity, while other prior art has discussed the backscattering cross-sectional amplitudes of p or s waves incident upon the cavity. With few exceptions, the numerical calculations are sparse, pertain only to a few metals, and are mostly restricted to the low frequency, i.e., Rayleigh, regime. Although some of the prior art has attributed the rapid oscillations in cross-sectional values to some sort of resonant phenomena, no attempt has been made to actually cast the analysis in some explicit resonant form. In effect, the amplitudes of backscattered waves returned by fluid-filled cavities in viscoelastic solids, when plotted as a function of frequency, exhibit so many rapid oscillations and complicated features that until very recently it did not appear possible to extract the physical information contained in them.